1. Field of the Invention
The present invention relates to an elastic wave device for use in, for example, resonators and band pass filters, and more particularly, to an elastic wave device including IDT electrodes covered with an insulating layer.
2. Description of the Related Art
Duplexers (DPXs) and RF filters for use in, for example, mobile communication systems are required to have wide band widths and satisfactory temperature characteristics. With respect to a surface acoustic wave device having a wide band width and satisfactory temperature characteristics, for example, International Publication No. WO 2006/011417 A1 discloses a surface acoustic wave device in which Al IDT electrodes embedded in a LiTaO3 substrate are covered with a SiO2 layer having a positive temperature coefficient of frequency (TCF). Like the surface acoustic wave device disclosed in International Publication No. WO 2006/011417 A1, embedding the IDT electrodes in the piezoelectric substrate provides a high electromechanical coupling coefficient (k2), thereby achieving a wide band width. Furthermore, covering the IDT electrodes with the SiO2 layer having a positive TCF achieves satisfactory temperature characteristics.
For an acoustic wave device with IDT electrodes that are embedded in a piezoelectric substrate, the IDT electrodes preferably have a higher density in order to achieve a higher electromechanical coupling coefficient (k2). From the viewpoint of reducing insertion loss, electrode fingers of the IDT electrodes made of a material with a low resistivity are preferably formed so as to have a large thickness to reduce the resistance of the electrode fingers.
However, in the case where IDT electrodes with a high density are formed so as to have a large thickness, the acoustic velocity of an elastic wave tends to be low. Thus, for example, in the case where an elastic wave device using an elastic wave in a high-frequency band, for example, a several hundred megahertz to several gigahertz band, is formed, it is necessary to disadvantageously reduce the pitch of electrode fingers of the IDT electrodes. A reduction in the pitch of the electrode fingers is liable to cause a reduction in the static electricity resistance of the elastic wave device and the occurrence of a ripple due to an elastic wave in a transverse mode.